1. Field of the Invention
The present invention relates to compositions used to cover, protect and insulate molten steel in a tundish during continuous casting of steel and the methods of using such compositions.
2. Background of the Prior Art
In continuous casting of molten steel, a tundish acts as a reservoir of molten steel between a ladle and caster in order to provide a continuous supply of molten steel into a mold. In a sequence casting procedure, anywhere from four to fifteen ladles of steel are continuously cast in a sequence. Molten steel from the ladle is poured into the tundish through a shroud and steel from the tundish is poured into one or more molds with a SEN tube. Fluid flow through the tundish is directed using dams, weirs, and baffles in such a way as to float out inclusions to the top and collect clean steel to flow into the mold.
In the past, an insulating layer of burnt rice hull ash (carbon and SiO.sub.2) was used to insulate the molten steel. This is a cheap way to insulate the steel in the tundish, but produces less than fully satisfactory steel and is often environmentally disruptive. With increasing need for cleaner steel and a safer working environment, slag producing tundish powders have been introduced. These powders, such as Atlantic Metal Corporation's TUNDOLITE.RTM. TU920, comprise primarily carbon and aluminum silicate (Al.sub.2 O.sub.3 -SiO.sub.2) and are effective in preventing the re-oxidation of steel in the tundish and are environmentally clean. Unfortunately, these compositions do not have the ability to absorb large quantities of Al.sub.2 O.sub.3 and do not insulate well.
In order to meet continuously increasing demands for high quality steel, recent investigations have centered on use of basic tundish powders with high CaO+MgO/SiO.sub.2 ratios. Although these powders have the ability to absorb Al.sub.2 O.sub.3, they have other production problems, such as crust formation that prevents free operation of tundish during sequence casting, and severe erosion of Al.sub.2 O.sub.3 /graphite SEN tubes.
To attempt to solve some of these problems, applicant investigated using calcium aluminate (CaO-MgO-Al.sub.2 O.sub.3) slag chemistries. These slags provided a basic slag cover but could not absorb large quantities of Al.sub.2 O.sub.3 without forming a crust and causing operational problems during sequence casting. This reaction is believed to be a function of the amount of aluminum oxide absorbed in the slag causing a increase in the melting temperature of the slag.
Accordingly, it is a primary object of the present invention to provide a cover for use in continuous casting of molten steel which effectively provides insulation of molten steel in the tundish, absorbs inclusions floating on top of the steel, protects the steel from re-oxidation, contamination and crusting, and resists wear on continuous casting refractories.
It is a further object of the present invention to provide such a cover for molten steel which comprises a basic tundish powder having the ability to absorb Al.sub.2 O.sub.3 without forming a crust due to increased melting point.
It is an additional object of the present invention to provide such a cover for molten steel which improves the cleanliness of steel without adversely affecting continuous casting operation.
It is another object of the present invention to provide such a cover for molten steel which delivers the above benefits while being simple to apply in the continuous casting process and relatively inexpensive to produce.
These and other objects of the present invention will become evident from review of the following specification.